The Siege of Belgrade, Battle of Belgrade or Siege of Nándorfehérvár was a military blockade of Belgrade that occurred from July 4–22, 1456. After the fall of Constantinople in 1453, the Ottoman Sultan Mehmed the Conqueror rallied his resources in order to subjugate the Kingdom of Hungary. His immediate objective was the border fort of the town of Belgrade (in Hungarian: Nándorfehérvár). John Hunyadi, the Count of Temes and captain-general of Hungary, who had fought many battles against the Turks in the previous two decades, prepared the defenses of the fortress.
The siege escalated into a major battle, during which Hunyadi led a sudden counterattack that overran the Ottoman camp, ultimately compelling the wounded Mehmed II to lift the siege and retreat. The battle had significant consequences, as it stabilized the southern frontiers of the Kingdom of Hungary for more than half a century and thus considerably delayed the Ottoman advance in Europe.
The Pope celebrated the victory as well, as he had previously ordered all Catholic kingdoms to pray for the victory of the defenders of Belgrade. This led to the noon bell ritual that is still undertaken in Catholic and old Protestant churches. The day of the victory, 22 July, has been a memorial day in Hungary ever since.
At the end of 1455, John Hunyadi began preparations for the defence of Belgrade. At his own expense, he provisioned and armed the fortress with a strong garrison under the command of his brother-in-law Mihály Szilágyi and his own eldest son László. Hunyadi then proceeded to form a relief army and an additional fleet of two hundred corvettes. The barons feared Hunyadi's growing power more than the Ottoman threat and left him entirely to his own devices.
An Italian Franciscan friar allied to Hunyadi, Giovanni da Capistrano, preached a crusade to attract peasants and local countryside landlords to Hunyadi's cause. The recruits were ill-armed, many with only slings and scythes, but they were highly motivated. The recruits came under Hunyadi's banner, the core of which consisted of smaller bands of seasoned mercenaries and a few groups of minor knights. All in all, Hunyadi managed to build a force of 25–30,000 men.